#14249: ApplicativeDo: Pattern matching on a bind forces a Monad constraint --------------------------------------+--------------------------- Reporter: mutantmell | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Keywords: ApplicativeDo | Operating System: Linux Architecture: x86_64 (amd64) | Type of failure: Other Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: --------------------------------------+--------------------------- Pattern matching on a bind in 8.2.1 confuses ApplicativeDo, and causes the type to have a Monad constraint where a Functor constraint would suffice. In GHC 8.0: {{{#!hs
:set -XApplicativeDo :t \x -> do { (a, _) <- x; pure (a+1) } \x -> do { (a, _) <- x; pure (a+1) } :: (Num b, Functor f) => f (b, t) -> f b }}}
In GHC 8.2: {{{#!hs
:set -XApplicativeDo :t \x -> do { (a, _) <- x; pure (a+1) } \x -> do { (a, _) <- x; pure (a+1) } :: (Num b1, Monad m) => m (b1, b2) -> m b1 }}}
-- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14249 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler